In the last decade, the discipline of positive psychology has developed a substantial number of interventions that are focused on increasing people’s happiness. There is evidence that interventions based on practising gratitude may enhance subjective well-being. To date, however, there are mixed findings regarding the efficacy of gratitude interventions. This Internet study investigated the effectiveness of a 3-day gratitude intervention programme based on the count-your-blessings approach on a sample of UK adults (N = 60). The outcome measure of the tested intervention was subjective well-being (SWB). Participants were measured at six points in time: before the intervention (Day 1), immediately after the intervention on each day (Day 2, Day 3, Day 4), one day after the 3-day gratitude intervention (Day 5), and at 1-week follow-up (Day 12). The results revealed that the gratitude intervention enhanced satisfaction with life and decreased negative affect in participants, and this positive effect of practising gratitude on SWB persisted over 1-week. In addition, the given intervention also momentarily increased individuals’ positive affect on Day 2. Interestingly, further analyses showed that the participants with low and average levels of self-esteem benefited the most from the gratitude intervention, regarding their satisfaction with life on Day 12 and negative affect on Day 5. Therefore, psychologists should stay sensitive to self-esteem as a moderator in future research. Theoretical and practical implications of these findings are discussed.